So is the capital gains tax rate really so insignificant? Perhaps. Back in August, the New York Times ran a piece on this very topic. Warren Buffett, for one, claimed that the tax rate on investment income doesn't make much of a difference to actual investors: "I have worked with investors for 60 years and I have yet to see anyone -- not even when capital gains rates were 39.9 percent in 1976-77 -- shy away from a sensible investment because of the tax rate on the potential gain. People invest to make money, and potential taxes have never scared them off."

Meanwhile, Troy Kravitz and Len Burman of the Urban Institute have shown that, over the past 50 years, there's no correlation between the top capital gains tax rate and U.S. economic growth -- even if you allow for a lag of up to five years. "Moreover," they add, "any effect is likely small as capital gains realizations have averaged about 3 percent of GDP since 1960 and have never been more than 7.5 percent."

Statement from Citizens For Tax Justice (bold is mine):

"It's an indictment of the federal tax code that a man of Mitt Romney's wealth could pay a federal tax rate as low as 10 percent. While he chose to forgo deductions for charitable contributions in order to keep his "commitment to the public that his tax rate would be above 13 percent," bringing his rate up to 14 percent for 2011, it is still outrageous that the code allows such a low rate.

"He also takes advantage of a special low rate on investment income. The preferential rate on capital gains and dividends saved Mitt Romney a whopping $1.2 million in taxes in 2011, cutting his tax bill almost in half.  He would have paid $3.1 million in taxes without that special treatment. And much of his low-rate income is really compensation from Bain Capital that should have been taxed like regular wages or salary, but is disguised as capital gains using the "carried interest" loophole.

"Romney also paid $675,000 under the Alternative Minimum Tax (AMT). If his own tax plan, which eliminates the AMT, had been in place in 2011, he would have saved himself an additional $675,000, or one third of his entire federal tax bill, and reduced his effective rate to 9 percent.

"Also notice that Mitt Romney's tax return for 2011 is almost twice as long as it was in 2010. It is 379 pages long, and 250 pages are foreign entity disclosure forms. Put simply, that's 250 pages about his offshore investments.

"Further, the summary provided by his lawyer is playing games by averaging Romney's 20-year tax rate. Including the years 1992-97 skewed his rate upwards because during those years, the capital gains rate was 28 percent instead of the 15 percent it is now. If they'd averaged only the last 15 years, his rate would have been much lower.

"And one final point is that Romney continued to work and make lots of money even when his capital gains tax rate was almost double the current rate, the rate he wants to retain.  Yet he says that the low capital gains rate is essential to incentivizing rich people to do what they do.  How does he explain that?"